1. Field of the Invention
The present invention is in the field of electrical instruments and more specifically relates to a calibrator for calibrating flow meters of the electromagnetic type. The calibrator is noteworthy in that it permits the calibration to be done in the field and without the use of a power supply or a supply of water.
2. The Prior Art
The type of flow meter which the present invention calibrates is exemplified by the flow meter described in U.S. Pat. No. 4,389,898 issued June 28, 1983 to Long, et al.
FIG. 1 herein is derived from FIG. 4a of the Long, et al. patent and shows in diagrammatic form a side view of a typical electromagnetic flow meter. As shown in
FIG. 1, the flow meter includes two U-shaped magnets 2, 4, and a common energizing coil 6 that is connected to a source of alternating or repetitive current. The coil 6 magnetizes the magnet cores establishing a fluctuating magnetic field indicated by the lines of flux 12. The sensor also includes the electrodes 8, 10 which are electrically isolated from the coil. In the space between the electrodes 8, 10 the magnetic field is directed predominantly in the direction indicated by the arrow, that is, perpendicular to a line joining the electrodes. The fluid, which could be water or some other liquid or a gas, flows perpendicular to the page, that is, perpendicular to both the magnetic field and the line joining the electrodes 8, 10. Under these conditions and in accordance with Faraday's principle, an electromotive force is developed between the electrodes 8, 10, and the magnitude of this emf is proportional to the flow velocity. Thus, the flow velocity is measured by measuring the voltage across the electrodes 8, 10. Clearly, if an alternating current is applied to the coil 6, this voltage also will be an alternating voltage and its amplitude will be proportional to the flow rate.
There are three main sources of error of such an instrument. First, there may be a zero offset by which is meant the sensor reads a non-zero velocity when placed in still water. Typically, zero offset is checked by placing the sensor in a bucket of water and taking a reading after the water has come to rest.
A second type of error is a deviation of the magnetic drive level from its desired value. Typically, this is caused when the circuit that drives the coil 6 of FIG. 1 does not provide a constant output. As a result, the strength of the magnetic field may deviate from its proper value. Normally, this is checked by measuring the output of the drive circuit. This measurement is complicated by the magnetic field produced by the coil, assuming the measurement is made with the coil connected to the drive circuit.
A third type of error is caused by fluctuation in the gain of the amplifier used in measuring the potential difference between the electrodes 8, 10. Normally, the gain of the amplifier is adjusted to compensate for variations in all of the other factors in order to bring the sensor into calibration. Typically, at the factory or special test facility, the sensor is placed in a stream of water that is moving at a precisely determined velocity, and with the sensor in the stream, the gain of the amplifier is adjusted until the instrument reads the known velocity. The technique, of course,is not usable in the field, because of the lack of a stream of known velocity.
In a typical sensor, the coil 6 is energized with a 15 Hz signal. Such an instrument is capable of measuring flow velocities as small as 0.05 feet per second with a repeatability of 0.01 feet per second.
Thus, it is seen that the conventional mode of measuring zero offset requires a bucket of still water, while the conventional way of calibrating the sensor at higher flow velocities requiries a special test facility in which the velocity of the liquid can be maintained at a constant known velocity. Accordingly, no practical method is known for calibrating electromagnetic flow meters under field conditions, and it is this need that the present invention is intended to fill.